Field of the Invention
The present invention relates to a one-way clutch and a brake control device for a magnetic tape drive apparatus, and more particularly to a one-way clutch and an apparatus incorporating this one-way clutch. The one-way clutch transmits the rotation of a rotating shaft only when the shaft rotates in a particular direction, the shaft being rotatable in forward and reverse directions.
With conventional magnetic recording/reproducing apparatuses such as a VTR, a reel disc is driven in rotation so that a reel placed on the reel disc is rotated to take up the magnetic tape. The tape take-up operation requires a few kinds of brakes used for their specific tasks.
A light braking force is applied to the rotating reel discs during, for example, the rewinding operation of the magnetic tape. A large braking force is applied to the rotating reel discs to bring the discs into instant stop when the tape should be stopped. The braking device for generating this large braking force is not activated during the rewinding operation but activated upon a stop command at the end of the rewinding operation.
In order to stop the tape at a desired position, a braking device should be activated as quickly as possible. For instantly applying a braking force, a mechanism referred to as one-way clutch is conventionally used.
FIGS. 8 and 9 illustrate a brake control device that controls a braking operation for a reel disc of a magnetic recording/reproducing apparatus. FIG. 8 illustrates a relevant portion of the construction of a conventional magnetic tape drive mechanism when a brake force is acting on a reel 8. FIG. 9 is a cross-sectional view taken along lines I--I of FIG. 8.
A brake member 1 is rotatably supported on a shaft 10 and has a brake pad 12 and a projection 13 located offset from the shaft 10. The brake member is urged by a tension spring 11 in a direction shown by arrow D. The urging force of the spring 11 causes the brake member 1 to rotate clockwise (shown by arrow E) about the shaft 10, so that the brake pad 12 is pressed against the side surface of the reel disc 8. A frictional force developed between the brake pad 12 and the reel disc 8 applies a braking force to the reel disc 8. When the reel disc 8 is rotating in a direction shown by arrow G, a light brake force is applied to the reel disc 8. When the reel disc 8 is rotating in a direction shown by arrow H, a full brake force is applied to the reel disc 8.
A brake-disabling member 2 is a plate cam formed with a cam portion 2b (beveled portion) and is laterally movable in directions shown by arrows A and C. The brake-disabling member 2 is urged by a tension spring 20 in the direction shown by arrow C and abuts a stopper, not shown. When the brake-disabling member 2 is moved in the directions shown by arrow A and C in FIG. 8, the projection 13 engages and moves along the cam portion 2b, so that the brake member 1 is rotated clockwise (shown by arrow E) or counterclockwise (shown by arrow F). The rotation of the brake member 1 in the direction shown by arrows C or A causes the brake pad 12 to move into or out of friction engagement with the reel disc 8.
Referring to FIG. 8, the projection 13 is in contact with a recess 2a (flat portion) of the brake-disabling member 2 where the brake member 1 has rotated fully in the direction shown by arrow E and the brake pad 12 is pressed against the reel disc 8. When the brake-disabling member 2 is moved in the direction shown by arrow A, the cam portion 2b of the brake-disabling member 2 pushes the brake member 1 out of the way so that the brake member 1 rotates in the direction shown by arrow F. Thus, the brake pad 12 is moved completely out of engagement with the reel disc 8.
The tension spring 20 biases at all times the brake-disabling member 2 in a direction shown by arrow C. Therefore, when the brake-disabling member 2 is released from where the brake-disabling member 2 has moved fully in the direction shown by arrow A, the brake-disabling member 2 instantly moves in the direction shown by arrow C with the aid of the biasing force of the tension spring 20 till the brake-disabling member 2 abuts the stopper, so that the projection 13 slides along the cam portion 2b toward the recess 2a and correspondingly the braking member 1 quickly rotates in the direction shown by arrow E. Thus, the rotation of the reel disc 8 is quickly reduced or brought to a stop.
A trigger member 3 is rotatably supported on a pin 30 mounted to the brake-disabling member 2. One end of the trigger member 3 is coupled to the brake-disabling member 2 via a tension spring 31. Thus, if the trigger member 3 is rotated clockwise about the pin 30 and subsequently released, the trigger member 3 returns to its original position with the aid of the tension force of the tension spring 31. The trigger member 3 has an engagement portion 32 and an engagement projection 33 which are substantially diametrically opposite to each other with respect to the pin 30.
The drive member 9 is a cam having an engagement portion 91 that engages the trigger member 3 at the engagement projection 33, and is supported on a shaft 90 driven by a motor, not shown. When the rotating member 9 is rotated by the motor in a direction shown by the arrow B, the engagement portion 91 will move into abutting engagement with the engagement portion 33, pushing and move the trigger member 3 together with the brake-disabling member 2 in the direction shown by arrow A.
FIG. 10 is a partial view of the rotating vanes 60 shown in FIGS. 8 and 9. A one-way clutch includes a rotating shaft 50, rotating vane member 6, clutch spring 7, and trigger member 3. The shaft 5 is rotated in either forward direction or reverse direction by a motor, not shown.
The clutch spring 7 is a coil spring that is tightly fitted to the shaft 50. The clutch spring 7 has an inner diameter slightly smaller than the outer diameter of the shaft 50 and is tightly fitted to the shaft 50. Thus, when the shaft 50 rotates without a load, the clutch spring 7 rotates together with the shaft 50 in the forward or reverse direction.
The rotating vane 6 is a hollow cylinder through which the shaft 50 extends so that the rotating vane is rotatable with respect to the shaft 50. The rotating vane 6 is formed with a slit 61 therein that extends parallel to the axis of rotation of of the shaft 50.
A free end 70 of the clutch spring 7 projects through the slit 61 radially outwardly of the rotating vane 6, thereby transmitting the rotation of the clutch spring 7 to the rotating vane 6. Thus, when the shaft 50 rotates without a load, the rotating vane 6 rotates together with the shaft 50 in the forward or reverse direction.
The rotating vane member 6 includes one or more vanes 60. When the trigger member 3 is moved in the direction shown by arrow A as shown in FIG. 8 so that the engagement portion 32 enters the rotation path of the vanes 60, the vane 60 moves into engagement with the engagement portion 32.
When the shaft 50 is rotating in a direction shown by arrow L and the engagement portion 32 interferes with the vane 6, the clutch spring 7 becomes loose with respect to the shaft 50. Therefore, the shaft 50 rotates continues to rotate while the clutch spring 7 and rotating vane 6 will not rotate.
When the shaft 50 is rotating in a direction shown by arrow J and the engagement portion 32 interferes with the vane 60, the clutch spring 7 holds the shaft 50 more firmly thereby transmitting the rotation of the shaft to the rotating vane 60. Thus, the engagement portions 32 is pushed out of the rotation path of the vanes 60. In this manner, the clutch spring 7 transmits the rotation of the shaft 50 to the rotating vane 6 only when the shaft 50 rotates a specific direction.
FIGS. 11-14 illustrate the operation of the one-way clutch device. FIG. 11 shows the engagement portion 91 and the projection 33 when the engagement portion 91 is rotated in the direction shown by arrow B, where the engagement portion 91 and the projection 33 are not in engagement with each other yet. The trigger member 3 remains at rest till the drive member 91 driven by a motor is not in engagement with engagement projection 33. Thus, the clutch spring 7 transmits the rotation of the shaft to the rotating vane 6 so that the rotating vane 6 rotates in the direction shown by arrow J.
FIG. 12 illustrates the drive member 9 when the engagement portion 91 rotating in the B direction pushes the trigger member 3 in the direction shown by arrow A. The engagement portion 32 is pushed at 33 by the engagement portion 91 to enter the rotation path of the vane 60. The engagement portion 32 engages the vane 60 to apply a rotational load so that the clutch spring 7 becomes loose with respect to the shaft 50. Thus, the clutch spring no longer transmits the rotation of the shaft 50 and the rotating vane 6 stops rotating.
FIG. 13 illustrates the shaft 50 rotating in the L direction when the engagement portion 91 rotating in the B direction pushes the trigger member 3 in the direction shown by arrow A. The clutch spring 7 firmly holds the shaft 50 to transmit the rotation of the shaft 50 to the rotating vane 6. The engagement portion 32 is pushed at 33 by the engagement portion 91 to enter the rotation path of the vane 60. The engagement portion 32 engages the vane 60 to apply a rotational load. However, the clutch spring firmly holds the shaft 50 to continue to transmit the rotation of the shaft 50. As a result, the vane 60 repels the engagement portion 32 out of its rotation path in a direction of centrifugal force. The trigger member 3 rotates about the shaft 30 so that the engagement projection 33 pops out of engagement with the engagement portion 91.
FIG. 14 illustrates the trigger member 3 shortly after the trigger member 3 has been repelled out of the rotation path of the vane 60. Since the trigger member 3 has become out of engagement with the engagement portion 91, the trigger member 3 instantly moves together with the brake-disabling member 2 in the direction shown by arrow C due to the urging force applied by the tension spring 20, thereby applying a large braking force to the reel disc 8. The trigger member 3 also rotates in a direction shown by arrow P and returns to its original position shown in FIG. 11 since the tension spring 31 pull back the trigger member 3 as shown FIG. 8.
With the aforementioned conventional one-way clutch apparatus, the movement of the rotating vane 6 along the shaft 50 is restricted by bearings. Therefore, when the engagement portion 32 collides with the vane 60, the trigger member 3 may be caught between the vane 60 and the engagement portion 91, i.e., "locked condition".
The rotating vane 6 needs at least one vane 60 that engages the engagement portion 32. However, if the rotating vane 6 has only one vane 60, there is a problem. That is, depending on the rotational position of the vane 60, the rotating vane 6 may need to rotate through almost one complete rotation before the engagement portion 32 engages the vane 60 when the shaft is rotated in a reverse direction. This causes a loss in time when rotating in a reverse direction.
If the rotating vane 6 has many vanes 60, the loss in time during the reverse rotation of the shaft 50 decreases, so that the brake device is activated more quickly.
In this case, an increased number of vanes 60 causes an increased chance of the vane 60 colliding with the engagement portion 32, thus eventually increasing the chance of being locked.